Optical routers based on surface plasmons

ABSTRACT

An optical router includes a substantially transparent dielectric layer, a metal layer with a first side that forms an interface with the dielectric layer, an input optical waveguide, and an output optical waveguide. The input optical waveguide is positioned to illuminate a portion of the interface. The interface is configured to cause light from the input waveguide to produce surface plasmons that radiate light to the output optical waveguide.

This application is filed as a divisional of co-pending patentapplication Ser. No. 10/350,780 filed Jan. 24, 2003, the entiredisclosure of which is hereby incorporated by reference herein for allpurposes.

BACKGROUND

1. Field of the Invention

The invention relates to wavelength-selective optical routers.

2. Discussion of the Related Art

All-optical routers do not convert an input optical signal into anintermediate electrical signal prior to transmitting an output opticalsignal. By avoiding conversions between light and electrical signals,all-optical routers are typically able to perform routing more rapidlythan non all-optical routers. All-optical routers are also typicallysimpler devices than non all-optical routers because of the absence ofconversions between optical and electrical signals. Higher speed andlower complexity has made conventional all-optical routers widelypreferred over non all-optical routers.

Though all-optical routers have desirable operating speeds, fabricatingsuch devices is often complex and expensive. For example, manyall-optical routers use arrayed waveguides. Fabricating suitable arrayedwaveguides usually requires high precision methods and expensive masks.For that reason, all-optical routers are often costly. It is desirableto have optical routers that route at high speeds, but do not requirecostly and complex fabrication processes.

SUMMARY

The various embodiments provide optical routers that performoptical-to-electrical and electrical-to-optical signal conversions. Theoptical-to-electrical conversion creates a surface plasmon thatpropagates between input and output ports. The surface plasmon'spropagation direction depends on the frequency of the input opticalsignal. The directional dependence of the propagation of the surfaceplasmon provides a wavelength-dependent routing of received opticalsignals. Since creation. annihilation, and propagation of surfaceplasmons are rapid processes, these non all-optical routers operate athigh speeds.

An optical router includes a substantially transparent dielectric layer,a metal layer with a first side that forms an interface with thedielectric layer, an input optical waveguide, and an output opticalwaveguide. The input optical waveguide is positioned to illuminate aportion of the interface. The interface is configured to cause lightfrom the input waveguide to produce surface plasmons that radiate lightto the output optical waveguide.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is cross-sectional view illustrating the kinematics of thecreation of a surface plasmon by a photon at a planar metal-dielectricinterface;

FIG. 2 illustrates dispersion relations for surface plasmons and photonsat the interface of FIG. 1;

FIG. 3 is a top view of an interface without translation invariancealong itself;

FIG. 4 illustrates how dispersion relations enable creation of a singlesurface plasmon in the incidence plane of a photon;

FIG. 5 illustrates how incident photons produce surface plasmons withmore than one momentum when the projected photon momentum isperpendicular to a reciprocal lattice vector;

FIG. 6 shows electric field intensities that are induced on a back sideof a metal layer of FIG. 3 in response to a front side of the metallayer being illuminated;

FIGS. 7A–7E are gray-shade plots of electric field intensities along theback side of the metal layer of FIG. 3 for various light configurationsincident on the metal layer's front side;

FIGS. 8 and 9 are respective cross-sectional and back side views of anoptical router that performs frequency-selective optical routing;

FIG. 10 illustrates a method for optically routing light with theoptical router of FIGS. 8 and 9; and

FIGS. 11 and 12 show a simple optical router that uses an end face of afiber bundle to perform frequency-selective optical routing.

Like reference numbers refer to functionally similar elements in thevarious Figures.

DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS

Various embodiments provide non all-optical routers. The new opticalrouters perform intermediate optical-to-electrical andelectrical-to-optical conversions that involve the creation andannihilation of surface plasmons at a metal surface. The surfaceplasmons are created by photons received at input optical port. Theannihilation of the surface plasmons creates photons at output opticalports. These optical-to-electrical and electrical-to-optical signalconversions are single particle events that are constrained by particlekinematics at a planar interface.

FIG. 1 illustrates kinematics for creation of a surface plasmon at aplanar interface 4 between a metal layer 6 and a dielectric substrate 8.A unit vector, î, is normal to the metal-dielectric interface 4, and aunit vector, û, is parallel to the metal-dielectric interface 4. Aphoton of momentum, K, and frequency, ω, is incident on the front sideof the metal-dielectric interface 4. The incident photon makes an angleθ with the unit vector, î, normal to the metal-dielectric interface 4.When an incident photon creates a single surface plasmon, the surfaceplasmon has the same frequency ω and a momentum K_(sp). The surfaceplasmon propagates along a direction defined by the unit vector û. Thesurface plasmon's momentum K_(sp) may be completely in, partially in, orperpendicular to the photon's incidence plane as described below.

The energy and momenta of photons and surface plasmons satisfydispersion relations. The dispersion relation for a single surfaceplasmon is:K _(sp)(ω)=(ω/c)([∈_(m)(ω)∈_(d)]/[∈_(m)(ω)+∈_(d)])^(1/2) û,Here, ∈_(m)(ω) and ∈_(d) are dielectric constants of the metal anddielectric on the two sides of the interface 4, and c is the speed oflight. The dispersion relation for the projection, K_(x)(ω), of thephoton's momentum, K, onto the metal-dielectric interface 4 is:|K _(x)(ω)|=(ω/c)(∈_(d))^(1/2)sin(θ).The magnitude of the projection, |K_(x)|, depends on the photon'sincidence angle.

FIG. 2 illustrates dispersion curves relevant to creation of a singlesurface plasmon and by a single photon at a metal-dielectric interfacethat is translation invariant along itself. At such an interface, boththe energy and the projection of the momentum along the interface areconserved. The conservation of energy requires that the annihilatedphoton and created surface plasmon have the same frequency, ω. Theconservation of the projection of the momentum along the interfacerequires that K_(x) and K_(sp) be parallel to the same unit vector û,and further requires that K_(x)(ω)=K_(sp)(ω).

The momenta of surface plasmons propagating parallel and anti-parallelto the projected photon momentum K_(x)(ω) are given by branchesK_(sp+)(ω) and K_(sp−)(ω). The condition K_(x)(ω)=K_(sp)(ω) can only besatisfied if one of the photon's limiting dispersion curves K_(x±)(ω),i.e., K_(x±)(ω)=±(ω/c)(∈_(d))^(1/2), crosses the surface plasmon'sdispersion curve. Since neither of the limiting forms K_(x±)(ω) crossesK_(sp+)(ω) or K_(sp−)(ω), crossing points do not exist between thephoton and surface plasmon dispersion curves. For that reason, a singlephoton cannot be converted into a single surface plasmon at ametal-dielectric interface that is translation invariant along itself.

The inventors realized that a single photon can however, be converted toa single surface plasmon at an interface that is not translationinvariant along itself. On such interfaces, dispersion curves do nothave to cross for resonant production of surface plasmons.

FIG. 3 shows a metal-dielectric interface 4′ without translationinvariance along itself. Translation invariance is broken by a finiteregular array 12 of deformations 14 in the interface 4′. Exemplarydeformations 14 include dimples and holes in the material on either sideof the interface 4′. The deformations 14 define a regular lattice ofsites at lattice positions P. The lattice positions P are defined byP=mb₁+nb₂ where b₁ and b₂ are fundamental lattice vectors and n and mare integers. Associated with the regular lattice 12 of deformations 14is a reciprocal lattice with fundamental vectors G₁ and G₂. Thereciprocal lattice vectors are defined the relations:G _(i) ·b _(j)=2πδ_(ij).The reciprocal lattice has a role in the creation of surface plasmons onthe metal-dielectric interface 4′.

The regular array 12 reduces full translation invariance along themetal-dielectric interface 4′ to a discrete subgroup. The discretesubgroup includes translations by integer multiples of the array'slattice fundamental vectors b₁ and b₂. Due to the reduced translationinvariance, energy-momentum conservation takes a modified form for aphoton incident on portions of metal-dielectric interface 4′ inside thefinite regular array 12. For such photons, resonant conversion of asingle photon into a single surface plasmon is constrained by themodified momentum conservation relation:K _(sp)(ω)=K _(p)(ω)+rG ₁ +sG ₂.Here, “r” and “s” are integers, and G₁ and G₂ are the above-describedreciprocal lattice vectors for the finite-size regular array 12.

The modified conservation law is valid for large enough arrays 12 toproduce approximate translation symmetry inside the array 12. Typically,this would require that the array 12 have, at least, four or more rowsand/or columns of substantially identical deformations 14. Exemplaryarrays 12 are invariant under discrete translations in 1 or 2independent lattice directions.

Unlike the momentum conservation rule for a translation invariantinterface, the above modified momentum conservation rule has solutionsin which a single incident photon creates a single surface plasmon. Thecreated surface plasmons propagate in directions correlated toreciprocal lattice vectors of the regular array 12.

FIG. 4 shows particle dispersion curves relevant to creating a surfaceplasmon with a momentum in the photon's plane of incidence on themetal-dielectric interface 4′ of FIG. 3. The dispersion curves show thatan incident photon of frequency ω₊ will create a surface plasmon ofmomentum K_(sp)(ω₊) where K_(sp)(ω₊)=K_(x)(ω₊)+G₁. The dispersion curvesalso show that an incident photon of frequency ω⁻ will create a surfaceplasmon of momentum K_(sp)(ω⁻) where K_(p)(ω⁻)=K_(x)(ω⁻)−G₁. Thefrequencies ω₊ and ω⁻ are different, and the created surface plasmonshave momenta K_(sp)(ω₊) and K′_(sp)(ω⁻), which are anti-parallel to eachother. For these reasons, incident photons of frequencies ω₊ and ω⁻produce surface plasmons that propagate in opposite directions on theinterface 4′. The reduced symmetry of the metal-dielectric interface 4′makes the propagation direction of created surface plasmonsfrequency-dependent. The frequency dependence of the propagation ofsurface plasmons can produce wavelength-dependent optical routing (seebelow).

The modified momentum conservation relation also has solutions for whichK_(sp)(ω) and K_(x)(ω) are not parallel provided that the array 12 hastwo independent lattice directions. FIG. 5 shows an exemplary solutionfor which K_(sp1)=K_(x)+G₁ with K_(x) perpendicular to G₁. This solutionrequires that |K_(sp)| and |K_(x)| solve the photon and surface plasmondispersion relations for the same frequency ω. Since K_(x) and G₁ areperpendicular, the momentum |K′_(sp1)| and |K_(x)| defined byK′_(sp1)=K_(x)−G₁ also solve the respective photon and surface plasmondispersion relations for the same frequency ω. Thus, photons withprojected momenta K_(x) perpendicular to reciprocal lattice vector G₁can create a surface plasmon with different momenta.

Situations where a photon can create surface plasmons with severalmomenta values are not efficient if only one of the produced types ofsurface plasmon is able to make output photons. To avoid producingsurface plasmons with more than one momentum value, the photon'sincidence plane photons can be selected so that the photons haveprojected momenta K′_(x) not orthogonal to the reciprocal latticevectors G₁ and G₂ as shown in FIG. 5.

FIG. 6 shows electric field intensities produced at a back side of anexemplary metal layer described by FIG. 3 in response to illuminatingthe exemplary metal layer's front side with 760 nanometer (nm) laserlight. The front side of the metal layer forms a metal-dielectricinterface with a dielectric fused silica substrate. In FIG. 6, measuredelectric field intensities are proportional to measured photon countingrates. During acquisition of the data of FIG. 6, the metal layer wasilluminated by collimated laser light that was incident on the frontside of the metal layer at an angle of about 50° to the layer's normalvector. The laser light is incident on a 6×7 rectangular array of holesin the metal layer. The holes have diameters of about 200 nanometers(nm) and form a finite regular lattice whose lattice vectors b₁ and b₂have lengths of 840 nm and 950 nm, respectively. During theillumination, electric field intensities were measured by scanning afinely pulled end of an optical fiber along the backside of the metallayer and counting photon production rates in the optical fiber.

In FIG. 6, the 2D plot of electric field intensities has two prominentfeatures. A first feature is an array 18 of large peaks. The array 18 oflarge peaks is located over the portion of the metal layer pierced by aregular cubic array of holes. The array 18 of large peaks results fromthe array of holes in the gold layer. A second feature is a straightline 20 starting end at the array of holes in the gold layer. Along theline 20 electric field intensities have elevated values due toevanescent electrical fields of surface plasmons. The straight line 20shows that photons incident on a metal-dielectric interface with aregular array of deformations will produce surface plasmons with alimited number of propagation directions. For the experimental layer ofgold, the produced surface plasmons propagate without significantdissipation over distances of 20 microns or more.

FIGS. 7A–7E are gray-shade plots that show electric field intensitiesalong the back side of the same metal-dielectric interface already shownin FIGS. 3 and 6. Inserts in the FIGS. 5A–5E illustrate the modifiedmomentum conservation relations associated with creating surfaceplasmons. In FIGS. 5A–5E, the electric field intensities were producedby illuminating the front side of the interface with various laser lightconfigurations. The lighter and darker shaded portions of the plotscorrespond to respective higher and lower electric field intensities. Inthe plots, the electric field intensity has a cubic array 22 of peakvalues over the array of deformations, i.e., holes, in the metal layer.In the plots, other relatively high values of the electric fieldintensity are associated with the evanescent electric fields of surfaceplasmon jets 24, 26, 28. The surface plasmons of the jets 24, 26propagate outward from the array 22 of deformations associated with thearray 22 of peaks. The final jet 28 is produced by surface plasmons thatare propagating outward from another array of holes in the metal layer(not shown).

A comparison of FIGS. 7A and 7B shows that varying the frequency ofincident laser light enables selective excitation of surface plasmonjets. FIGS. 7A and 7B illustrate jets of surface plasmons excited bylaser light with the same incidence direction, but different frequenciesω and ω′. At the frequency ω, the laser light strongly excites surfaceplasmons of momenta K_(sp)(ω) where K_(sp)(ω)=K_(x)(ω)−G₁. These surfaceplasmons belong to the jet 24 of FIG. 5A. At the frequency ω′, the laserlight excites much fewer surface plasmons with momenta K_(sp)′(ω) equalto K_(x)(ω′)−G₁. This is seen by the lower intensity of evanescent lightalong the jet 24 in FIG. 7B. At the frequency ω′, the laser light muchmore strongly excites surface plasmons of momenta K_(sp)′(ω) whereK_(sp)′(ω)=K_(x)(ω)−2G₂. This is seen by the relatively higher intensityof evanescent light in the jet 26 of FIG. 7B. Thus, conversion ofphotons into surface plasmons can be substantially switched between thejets 24 and 26 by changing the frequency of the incident laser lightbetween ω and ω′.

A comparison of FIGS. 7B and 7D shows that varying the incidence angleof the laser light enables selective excitation of surface plasmon jets.FIGS. 7B and 7D illustrate jets of surface plasmons excited by laserlight of the different respective incidence angles θ and θ′ and the samefrequency ω′. As the light's incident angle varies from θ to θ′, the jet26 turns off while the jet 24 remains at approximately the sameintensity. As the light's incidence angle is varied further in the samedirection without changing frequency, both jets 24 and 26 substantiallyturned off as shown in FIG. 7E.

A comparison of FIGS. 7B and 7C shows that varying the polarization oflaser light also enables selective excitation of surface plasmon jets24, 26. FIGS. 7B and 7C show surface plasmon jets excited by laser lightwith the same frequency and incidence direction but differentpolarizations. In FIG. 7B, the electric field of the incident light isnot orthogonal to either jet 24, 26. For that reason, surface plasmonsare excited in both jets of FIG. 7B. In FIG. 7C, the electric field ofthe incident light is perpendicular to the direction of the jet 26. InFIG. 7C, the jet 26 is not excited, because surface plasmons arelongitudinal electron density waves and thus, are not excited byelectric fields perpendicular to their propagation direction. In theincidence configuration of FIG. 7B, only one polarization of theincident laser light excites surface plasmons in the jet 26, i.e., thepolarization parallel to the jet 26.

As FIGS. 6 and 7A–7E illustrate, a regular array of deformations enablesa metal-dielectric interface to produce optical-to-electrical andelectrical-to-optical signal conversions that involve jets of surfaceplasmons. The directional dependence of the production of surfaceplasmons enables construction of optical routers that are based onphoton-to-surface plasmon and surface plasmon-to-photon conversions.Fabricating such non all-optical routers requires simpler and oftenless-costly methods, but still enables high routing speeds due to theabsence of a need for circuitry to produce the conversions betweenoptical and electrical signals.

FIGS. 8 and 9 show an optical router 30 that performsfrequency-selective optical routing by creating jets of surfaceplasmons. The optical router 30 includes a dielectric layer 32 and ametal layer 34 located on the dielectric layer 32. The metal layer 34includes a metal such as gold, copper, silver, or aluminum and has athickness of 1–5 times a skin depth of the wavelength of light beingrouted. Exemplary dielectric layers 36 include hard dielectrics such asilica glass and soft dielectrics such as air. The front side of themetal film 34 forms a planar interface 36 with the dielectric layer 32.

The metal-dielectric interface 36 includes first, second, and thirdregular arrays 38–40 of deformations 42. The deformations 42 may beholes, dimples, straight vias, bumps, or straight ridges in the metallayer 34, the dielectric substrate 32, or both. Exemplary deformations42 traverse the entire metal layer 34. Preferably, each array 38–40includes a series of four or more substantially identical and equallyspaced rows of deformations 42 along one direction. Such array sizesproduce an approximate discrete translation symmetry inside the arrays38–40 thereby enabling resonant generation of surface plasmons when theabove-described modified momentum conservation relations are satisfied.Some arrays 38–40 also include a series of four or more substantiallyidentical and equally spaced deformations 42 in each row therebyproducing approximate discrete translation symmetries in two independentdirections on the metal-dielectric interface 36.

In some embodiments, the first, second, third regular arrays 38–40 ofdeformations 42 are portions of a single regular array of identical andequally spaced deformations on the metal-dielectric interface 36.Exemplary regular arrays have the lattice types such as simple cubic,body-centered cubic, triangular, etc.

The optical router 30 includes an input optical waveguide 44, a lens 45,and an opaque layer 48. Exemplary input optical waveguides 44 includesingle or multi-mode optical fibers. The input optical waveguide 44 ispositioned to illuminate a portion of a front side of themetal-dielectric interface 36 with the input light signals being routed.The lens 45 collimates the input light from the input optical waveguide44 so that the input light is incident on the interface 36 at a singleangle θ″. The opaque layer 48 includes a window 50 over the first array38 of deformations 42. The window enables the input light signals tocreate surface plasmons in the first array 38, and stops input lightfrom directly creating surface plasmons in the other arrays 39–40 on themetal-dielectric interface. For this reason, the input light signalsonly create surface plasmons that propagate outward from the first array38.

The optical router 30 also includes first and second output opticalwaveguides 52, 54, and corresponding insertion lenses 53, 55 located atthe backside of the interface 36. Exemplary output optical waveguides52, 54 include single and multi-mode optical fibers. The lens 53 focuseslight radiated by surface plasmons propagating through array 39 into thefirst output optical waveguide 52. The lens 55 focuses light by surfaceplasmons propagating through the array 40 into the second output opticalwaveguide 54.

The arrays 39 and 40 lie along different angular directions with respectto a center defined in the first array 38. For that reason, a surfaceplasmon propagating outward from the first array 38 will propagatethrough, at most, one of the arrays 39 and 40 of deformations 42. Thus,individual surface plasmons created by input light signals only producean output light signal in one of the output optical waveguides 52 and54.

The relative orientations of the input optical waveguide 44 and thefirst array 38 cause input light of frequency ω₁ and ω₂ to createsurface plasmons that propagate towards the array 39 and the array 40,respectively. The orientation of the input optical waveguide 44 withrespect to both the metal-dielectric interface 36 and the first array 38insure that such created surface plasmons have momenta that satisfy theabove-described modified momentum conservation relations. For thatreason, input light signals frequency ω₁ and ω₂ resonantly create jetsof surface plasmons propagating towards the second array 39 and thethird array 40, respectively.

A preferred embodiment of the non all-optical router 30 causes inputlight of frequency ω₁ to only create surface plasmons that propagatetowards the second array 39 and causes input light of frequency ω₂ toonly create surface plasmons that propagate towards the third array 40.In one such configuration, the incidence plane of the input lightincludes a reciprocal lattice vector of the first array 38, and thearrays 39 and 40 are on opposite sides of the first array 38 asillustrated by FIGS. 4 and 8.

The relative orientation of the output optical waveguide 52 with respectto both metal-dielectric interface 36 and second array 39 is designed sothat a surface plasmon propagating outward from the first array 38 willresonantly radiate light in the second array 39. Furthermore, theradiated light will be directed towards the output optical waveguide 52.Thus, surface plasmons arriving in the second array 39 efficientlyradiate photons at frequencies and momenta that satisfy previouslydiscussed modified conservation relations with respect to the secondregular array 39 of deformations 42. Such a relative orientation insuresefficient coupling of input optical energy of frequency ω₁ to the outputoptical waveguide 52.

Similarly, the relative orientation of the output optical waveguide 54with respect to both metal-dielectric interface 36 and second array 40is designed so that a surface plasmon propagating outward from the firstarray 38 will resonantly radiate light in the second array 40.Furthermore, the radiated light will be directed towards the outputoptical waveguide 54. Thus, surface plasmons arriving in the secondarray 40 efficiently radiate photons at frequencies and momenta thatsatisfy previously discussed modified conservation relations withrespect to the third regular array 40 of deformations 42. Such arelative orientation insures efficient coupling of input optical energyof frequency ω₂ to the output optical waveguide 54.

FIG. 10 illustrates a method 60 for optically routing light with the nonall-optical router 30 of FIGS. 8 and 9. The optical router 30 produces ajet of surface plasmons on a metal surface in response to receiving anoptical signal of frequency ω₁, or ω₂ from the input optical waveguide44 (step 62). The jet radiates an above-threshold optical signal to theoutput optical waveguide 52 in response to the input optical signalhaving the frequency ω₁ (step 64). The jet may also radiate an opticalsignal in the other output waveguide 54, but such an optical signal hasa below-threshold value. The jet radiates an above-threshold opticalsignal to the other output optical waveguide 54 in response to the inputoptical signal having the frequency ω₂ (step 66). The jet may alsoproduce another optical signal in the output waveguide 52, but such anoptical signal has a below-threshold value.

In addition to the optical router 30 of FIG. 8, various fiber-devicesare also able to provide frequency-selective optical routing that isbased on optical to electrical to optical conversions involving surfaceplasmons.

FIG. 11 shows a fiber bundle 70 that provides frequency-selectiveoptical routing based on surface plasmons. The fiber bundle 70 includesan input optical fiber 72 and a plurality of output optical fibers 74,76 for receiving light from the input optical fiber 72. In the fiberbundle 70, optical routing involves the propagation of surface plasmonsalong an end face 78 of the joined-fiber region 80 of the fiber bundle70.

FIG. 12 provides an end view of the joined-fiber region 80 of FIG. 11.In the joined-fiber region 80, an index matching medium 82 is disposedbetween ends of the optical fibers 72, 74, 76. The end face 78 of thejoined-fiber region 80 has been planarized and coated with a metallayer, e.g., a gold layer, to produce a planar metal-dielectricinterface 84. The planar metal-dielectric interface 84 includes an arrayof identical deformations 86. The deformations produce a 2-dimensionallattice periodicity along the end face 76. Exemplary deformations 86 aredimples in the dielectric or metal located on the two sides of theplanar interface 84. Exemplary arrays of deformations 86 are produced bya well-known methods involving mask-controlled etches after planarizingthe end face 78 of the joined-fiber region 80.

The pattern of deformations 86 is configured so that the input opticalfiber 72 is able to optically excite surface plasmons that propagatealong the planar interface 84. In particular, light of preselected firstand second input frequencies will excite surface plasmons thatselectively propagate to the first output fiber 74 and to the secondoutput fiber 76, respectively. The periodic array of deformations 86also enables the created surface plasmons to produce light that couplesto the output optical fibers 74, 76. Thus, such surface plasmons producefrequency-selective optical routing between the optical fibers 72, 74,76 via a process that involves optical-to-electrical-to-opticalconversions. The above discussion of modified momentum conservationrelations would enable one of ordinary skill in the art to design anarray of deformations 86 suitable for such optical routing.

In other fiber embodiments of optical routers, input and output opticalfibers optically side-couple to a planar metal-dielectric interface. Theoptical fibers have coupling regions where portions of their opticalcladding have been removed to enable optical cores of the fibers tooptically couple the metal-dielectric interface. The metal-dielectricinterface includes a periodic array of substantially identicaldeformations. Due to the array of deformations, the input optical fibercreates surface plasmons for selected input optical frequencies. Due tothe array of deformations, the output optical fibers receive lightradiated by surface plasmons that propagate through associated couplingregions on the metal-dielectric interface.

Referring again to FIGS. 8 and 11–12, some embodiments of the opticalrouters 30, 70 route optical signals based on incidence angle, incidencepolarization, or dielectric constant at the metal-dielectric interface36, 84 rather than based on optical frequency. In one such embodiment,the router 30 includes a mechanical device for varying the incidenceangle θ″ of light emitted from the input optical waveguide 44. In thisrouter 30, input light produces surface plasmons that radiate to thefirst or second output optical waveguide 52, 54 based on the selectedincidence angle θ″. In another embodiment, the router 30 includes avariable voltage source connected to the metal layer 34 and configuredto apply a voltage across the dielectric layer 32. The dielectric layer32 is electro-optically active so that the applied voltage can changethe layer's dielectric constant and thus, change the dispersion relationfor surface plasmons at the interface 36. In this router 30, input lightproduces surface plasmons that radiate to the first or second outputoptical waveguide 52, 54 based on the selected voltage across thedielectric layer 32. In another embodiment, the router 30 includes avariable optical polarization rotator that is located between the end ofinput optical waveguide 44 and the lens 45. In this router 30, inputlight produces surface plasmons that radiate to the first or secondoutput optical waveguide 52, 54 based on the selected incidentpolarization.

The invention is intended to include other embodiments that would beobvious to one of skill in the art in light of the description, figures,and claims.

1. An optical router, comprising: a substantially transparent dielectricsubstrate; a metal layer with a first side that forms an interface withthe dielectric substrate; an input optical waveguide positioned toilluminate a portion of the interface; an output optical waveguide; andwherein the interface is configured to cause light of a first wavelengthfrom the input waveguide to produce surface plasmons that radiate lightto the output optical waveguide; and wherein the portion of theinterface is configured to cause light of a second wavelength from theinput optical waveguide to create surface plasmons that exit the portionof the interface along a different direction than the surface plasmonsproduced by the light of the first wavelength.
 2. The apparatus of claim1, further comprising: a second output optical waveguide positioned toreceive light produced by the surface plasmons created by the light ofthe second wavelength.
 3. The apparatus of claim 1, further comprising aregular array of deformations along the interface, the array configuredto cause light from the input optical waveguide of a selected frequencyto produce the surface plasmons.
 4. The apparatus of claim 3, whereinthe regular array comprises a plurality of rows of deformations,adjacent ones of the rows being separated by substantially the samedistance.
 5. The apparatus of claim 3, further comprising anotherregular array of deformations along the interface, the another arraybeing configured to cause the surface plasmons to radiate light to theoutput optical waveguide.
 6. The apparatus of claim 3, wherein thedeformations are selected from one of holes in the metal layer and holesin the dielectric layer.
 7. The apparatus of claim 3, wherein thedeformations of the array are equally spaced in two independentdirections.
 8. The apparatus of claim 3, wherein the deformations of thearray are substantially identical.
 9. The apparatus of claim 3, whereinthe deformations of the array are equally spaced in two independentdirections.